1. Field of the Invention
The present invention relates to the separation and recovery of toluene diisocyanate (TDI) from the residues from its manufacture.
2. Description of the Prior Art
It is well known to this art that appreciable amounts of heavy compounds are formed during the manufacture of TDI. These amounts, which can vary depending on the precise nature of the manufacturing process employed, generally represent from 10 to 15% by weight, or even more. These heavy compounds are usually concentrated by distillation of TDI, typically to a concentration of 30 to 40% of TDI and from 60 to 70% of heavy compounds.
It is difficult to continue the distillation in order to obtain mixtures with higher concentrations of heavy compounds, because of the increase in the viscosity of the mixture. Other techniques must therefore be employed to recover the TDI present in a mixture of this type, to dispose of a combustible residue containing the heavy compounds in question and no longer containing any TDI, and to avoid both contamination with toxic vapors and costly losses of final product.
Various such techniques have already been proposed to this art, but each has at least one of the following disadvantages:
(a) it is noncontinuous;
(b) it is costly in raw material and/or in energy requirements;
(c) it is very difficult to implement on an industrial scale.
An analysis of these various proposed techniques of the specific disadvantages which they exhibit is set forth in the preamble of European Patent No. 0,000,463, which also proposes a specific solution to this problem, namely:
continuous treatment of residues from this manufacture in a stirred and scraped evaporator, at a temperature commencing at 100.degree.-130.degree. C., at a pressure of 666.5 to 2,666 Pa, a temperature which is then progressively raised to a temperature of 220.degree. to 260.degree. C. with a minimum residence time of 15 min in the evaporator, in order to evaporate the TDI while continuously removing the residues, especially by utilizing an extrusion system.
According to said '463 European patent, the process described, which enables good results to be achieved on an industrial scale, nevertheless exhibits the following disadvantages: satisfactory operation thereof demands strict controls both of the physicochemical conditions (pressure, temperature, residence time) and of the mechanical conditions (stirring, transport of the products of increasing viscosity in the evaporator, scraping).
In fact, the smallest deviation from these conditions is liable to cause either a polymerization which can progress until the residue sets solid in the apparatus, or a decomposition of TDI into gases which generate foam in the viscous residual mixture.
Also, the movement of the viscous residue along the wall of the evaporator must be controlled such that the temperature can be increased in step with the evaporation of the TDI, in order to complete said evaporation without running the risk of decomposition.
Furthermore, in use, this particular process is found to exhibit the following additional disadvantage. Its operation is feasible only in the case of certain categories of residues, namely, those which present an increasing viscosity profile in step with the progress of the evaporation of TDI. In the case of residues from industrial manufacture which do not exhibit a profile of this kind, the process in question cannot be applied.
Thus, need continues to exist for a process for the separation of the TDI present in the residues from its manufacture and which is capable of being carried out continuously, independently of the nature or of the precise origin of the residues to be treated, which is both efficient, simple to implement on an industrial scale and low in cost and which, where appropriate, gives rise to a final waste in a form which can be easily handled.